This invention relates to a process for continuously growing silicon bodies resulting from chemical vapor deposition upon seed rods. In another aspect the invention relates to the continuous deposition of single crystal silicon bodies from continuously pulled virgin slim rod formed in situ, followed by heating of the slim rod to temperatures which provide suitable surface conditions for contacting silicon halide-hydrogen reaction gases with the rod in a chemical vapor deposition chamber having in combination different gas curtains along the chamber inner wall, followed by removal of the resulting single crystal silicon body from the chamber. In yet another aspect the invention relates to a process for increasing the silicon deposition rate from silicon halide-hydrogen reaction gases within the chemical vapor deposition chamber environment, the increased silicon deposition rate resulting from increasing small percentages by weight of silane to the silicon halide-hydrogen reaction gases.
In the semiconductor industry, it is common to deposit material from the gaseous state onto a substrate for the purpose of forming various electronic devices. In some applications the material deposition from the gas is the same material as that from which the substrate is formed, while in other instances it is a different material from that from which the substrate is formed. As an example of the former, in the growth of silicon by vapor deposition techniques, it is common to position an elongated silicon filament between two graphite electrodes each of which extends through the end of a quartz container within which the filament is placed. A potential is impressed across the graphite electrodes causing a current to flow through the filament. The resistance of the filament to a temperature generally in excess of about 1100.degree. C.
A gas stream, which comprises a mixture of trichlorosilane and hydrogen and/or other silanes is introduced into the quartz chamber and after flowing along the longitudinal axis of the filament is withdrawn from the chamber. The gas stream, upon contacting the hot surface silicon filament, will react to deposit polycrystalline silicon on the filament, thus increasing the diameter of the filament. The reaction of the trichlorosilane and hydrogen may be generally illustrated by the following simplified formula: EQU SiHCl.sub.3 +H.sub.2 .fwdarw.Si+3 HCl
Gas flow through the quartz cylinder or reaction chamber is usually continued for several hours to increase the diameter of the filament, which may be one-tenth inch in diameter upon commencement of the deposition, to the diameter in excess of five inches. When the silicon rod has reached a desired diameter, the flow is terminated and the rod is removed from the reaction chamber. Material deposited on the silicon filament will be polycrystalline and therefore must be zone melted to produce a single crystalline material. Alternatively, the poly crystal rod may be melted in a crucible and a large single rod is "pulled" from the melt by way of a variety of apparatus such as a Czochralski puller.
In both commercially accepted methods of producing single crystal silicon for the electronics industry, that is by float zone or by Czochralski, the single crystal rod which is drawn from a melt in both cases is rotated and results from the pulling of the melt in the form of the single crystal rod. Such methods require considerable skilled technician monitoring as well as multiple furnaces requiring substantial energy for operation. Even under the best conditions, frequently the crystal is lost during the first stage which means that the rod being pulled converts to a polysilicon growth zone; thus terminating the growth procedure of the rod. Such commercial methods of producing remelt single crystal silicon rod materials are costly in time and effort and frequently produce irregularly shaped cylindrical rods requiring substantial premachining before slicing and conversion into wafers for use in the electronics industry.
Recent developments in the semiconductor industry have created a growing demand for low-cost single crystal silicon of extremely high purity, which is known as semiconductor grade silicon. Semiconductor grade silicon is used in the manufacture of semiconductor devices, such as transistors, rectifiers, solar cells, and the like. Processes are in use in the prior art producing single crystal silicon through the remelting of polycrystalline semiconductor grade silicon.
The prior art processes have demonstrated the technical and economic feasibility of producing high purity polycrystalline silicon of semiconductor quality by hydrogen reduction of silicon halides. All commercial semiconductor polycrystalline silicon presently being manufactured through chemical vapor deposition processes employ hydrogen reduction of trichlorosilane or silicon tetrachloride and the deposition of silicon on electrically heated silicon filament substrate.
This method relates to a method for producing high purity silicon or other semiconductor materials primarily for semiconductor device use and, in particular, to an improvement of the Siemen's process as described by Gutsche, Reuschel, and Schweickert in U.S. Pat. No. 3,011,877 and by Gutsche in U.S. Pat. No. 3,042,494. According to these prior art patents, elemental silicon is obtained in the form of cylindrical rods of high purity by decomposing silicon halides from the gas phase at a hot surface of the purified silicon filament, the preferred halides being the chlorides, silicon tetrachloride and trichlorosilane. These compounds become increasingly unstable at temperatures above 800.degree. C. and decompose in two ways: (1) After adsorption on a hot surface which can provide a substrate for heterogeneous nucleation, for example, when the silicon halide concentration in the gas phase is kept relatively low by adding hydrogen as a diluent, the hydrogen also acts as a reducing agent; (2) in the case of high halide concentration in the gas phase, homogeneous nucleation occurs and the resulting silicon forms a dust of extremely fine particle size which is unsuitable for further processing. Heterogeneous nucleation hence silicon deposition starts at about 800.degree. C. and extends to the melting point of silicon at 1420.degree. C. Since the deposition is beneficial only on the substrate, the inner walls of the decomposition chamber must not reach temperatures near 800.degree. C.
On the other hand, the deposition chamber wall temperature must not be much lower than about 500.degree. C. because a cold wall is an effective heatsink and can easily overtax the ability of the electronic current passage through the substrate filament. Increasing the current through the substrate is not possible as a consequence of the negative temperature coefficient of resistivity in silicon which causes the electronic current to flow preferentially through the center of the cross section of the filament creating and maintaining thus an overheated core. Filaments with surface temperatures of over 1300.degree. C., for example, have usually a molten core. One embodiment of U.S. Pat. No. 3,042,494 describes the wall temperature can be effectively controlled by varying the amount and speed of the air circulation over the outside of the decomposition chamber. However, as the deposition progresses, the filament grows into rods of great weight, diameter and surface area which gives off radiant energy at least 10% of which is absorbed by the quartz wall where the wall is absolutely clear. Much more energy is absorbed when the quartz does not transmit readily because of flaws within the quartz wall or just a general roughness of the quartz surface. Experience has indicated that the cooling air alone cannot satisfactorily solve the overall needs of silicon chemical decomposition chambers.
Epitaxially grown single crystal silicon by chemical vapor deposition (CVD) has been known since the early 1960's. It is also known to utilize volatile acceptor or doner impurity precursors during growth process of silicon; thus leading to the formation of electrically-active regions, bounded by junctions of varying thicknesses, carrier concentrations, and junction profiles. Vapor substrate growth systems are quite general and in principle applicable to systems in which are provided pertinent kinetic and thermodynamic conditions of satisfactory balance.
The growth of single crystal silicon from the vapor phase is dependent on several important parameters all of which interract with each other to some degree. These parameters can be described in part, for example, as substrate surface crystalographic orientation, the chemical system, reaction variables, such as concentration, pressure, temperature modification, and the appropriate kinetic and thermodynamic factors. A variety of reaction systems have been investigated; however, all have the common feature that a hot single crystal surface is exposed to an atmosphere which is thermally and/or chemically decomposable. The mechanism of the silicon-forming reaction is a function of the temperature of the substrate.
In addition to known CVD epitaxially-grown silicon, epitaxial CVD processes using trichlorosilane have been demonstrated to achieve conversion yields of 50% or better and that single crystal deposition does occur at linear growth rates. It is further known that p-n junctions with theoretical I-V characteristics have been grown by CVD indicating the superiority of CVD crystal silicon over melt grown silicon. However, practical application of chemical vapor deposition to the preparation of single crystal semiconductor bodies has not been extended to the preparation of rod shaped bodies but has been limited to the preparation of thin epitaxial films on substrate in wafer form. All attempts at preparing thick, i.e., more than a few thousandths of an inch single crystal bodies, have failed because of the unavailability of adequate methods to provide and/or maintain an absolutely contamination-free surface on the substrate seed and other mechanical-chemical problems. Similarly, all attempts at preparing rod shaped bulk single crystal bodies on electrically heated single crystal filaments consisting of the same material have failed, mainly because of the unresolved difficulties with preparing and maintaining an absolutely contamination-free substrate seed surface. In previous attempts at developing processes for the direct deposition of rod-shaped single crystal bodies, energy applications have failed since direct electrical means were insufficient to provide the temperature uniformity essential to deposit a flawless single crystal body of homogeneous composition. Both a perfect single crystal structure and homogeneous chemical composition are conditions without which a semiconductor device constructed by means of that single crystal body will not function properly.
In the growth of polycrystalline silicon or single crystal silicon from deposition of silicon resulting from silicon halide-hydrogen reaction gases, the purity of the reaction gas is critical and is obtained by careful fractional distillation and by the particular design of the apparatus which assures that all materials used in the apparatus construction are very pure and do not promote contamination of the silicon halide-hydrogen reaction gases or depositing silicon under conditions of deposition. These requirements restrict the practical choice of materials that can be used for the construction of apparatus for the preparation of semiconductor silicon to quartz, graphite, silicon carbide, silver, and the like. Silver must always be thoroughly water cooled in order to avoid chemical reaction. The silicon halides used most for the preparation of high purity silicon are silicon tetrachloride and trichlorosilane. These halides will undergo pyrolysis when in contact with the hot surface and deposit elemental silicon. To obtain reasonable and economical yields, however, an excess of hydrogen gas is added to the silicon halides vapor reaction feed gas. Because of its higher silicon content, trichlorosilane will deposit more silicon than silicon tetrachloride and is therefore the preferred material for the Siemens' process for the preparation of polycrystalline silicon. However, silicon tetrachloride is preferred for the preparation of thin epitaxial films of single crystal silicon.
Silicon halides with less than three chlorine atoms in the molecule like SiH.sub.2 Cl.sub.2 and SiH.sub.3 Cl in particular, deposit much more silicon per mole of silicon halide consumed in the reaction but are impractical because they are not readily available and thus less desirable economically.
When trichlorosilane (SiHCl.sub.3) or silicon tetrachloride (SiCl.sub.4) are used in the Siemens' process, the overall reactions are assumed to be EQU SiHCl.sub.3 +H.sub.2 .fwdarw.Si+3 HCl and 1. EQU SiCl.sub.4+ 2 HCl.fwdarw.Si+4 HCl 2.
For a silicon halide to hydrogen mole ratio of 0.05%, thermal dynamic equilibrium is reached at 1150.degree. C. when approximately 48% of the SiCl.sub.3 moles have reacted, or 24% of the SiCl.sub.4 moles. In practice, however, equilibrium is not reached in a flow-through system because the kinetics of the reaction limit the actual steady state silicon yields to about one-half of the equilibrium values, the hydrogen chloride desorption from the substrate surface being the rate controlling step in both reactions.
Any occurrence on the substrate surface that could accelerate the hydrogen chloride desorption movement of hydrogen chloride away from the surface would accelerate the deposition rate of silicon and improve the economics of the process. One proven way to accomplish this result is to improve the deposition rate by accelerating the desorption rate of hydrogen chloride by raising the temperature of the substrate surface. This approach is effective but only when relatively small concentrations of silicon halide in the reaction gas are employed. These low mole ratios are lean mixtures and result in high yield but low weight gains. Rich mixtures do not respond in the desired manner because of side reactions as follows: SiHCl.sub.3 .fwdarw.(SiCl.sub.2)+HCl and SiCl.sub.4 +Si.fwdarw.2 (SiCl.sub.2); the side reaction producing the radical (SiCl.sub.2) which is stable at the reaction temperature range and reduces the amount of reactive silicon halide available for absorption and reaction on the substrate surface. As a net effect, we see, in spite of the faster HCl desorption, less silicon being deposited as we increase the substrate surface temperature from, for example, 1150.degree. C. to 1250.degree. C. when molar ratios in excess of 0.05 in the trichlorosilane system and in excess of 0.01 molar ratio in the silicon tetrachloride system are utilized. In fact, at molar ratios of about 0.1 in the silicon tetrachloride system, the formation of (SiCl.sub.2) becomes the dominant reaction and silicon is removed from the substrate at about 1200.degree. C.
Another approach to improved deposition rates would be to use mixtures of silicon halides so that the overall silicon chlorine ratios increase. For example, silane (SiH.sub.4) offers itself as an effective diluent and having no chlorine in the molecule would improve the silicon to chlorine ratios of silicon halide reaction gas mixtures. Silane as such cannot be used readily as a starting reaction material for the Siemens' process. Silane is not stable and decomposes spontaneously at 400.degree. C. forming silicon and hydrogen. The silicon, unfortunately, forms a dust which is not suitable for further processing rather than a controlled deposition upon a seed rod. Only in greatly diluted reaction gas stream wherein hydrogen, helium, or the like is utilized in the presence of hydrogen chloride, can silane be used to prepare silicon in crystalline form. Particular application is therefore limited mostly to slow deposition rate processes which are used exclusively in the thin film preparation field.